[panel] Make clock configurable via gconf

Add keys for customizing the panel clock to the gconf schema and make
the clock use them. The settings are copied from gnome-panel's clock
applet, excluding all location/weather/appointment/... keys. In addition,
'internet' is no longer a supported value for the format key.

https://bugzilla.gnome.org/show_bug.cgi?id=600276